The present invention relates to an interface circuit, and more particularly, to the control system power supplied to an interface circuit which performs data transmission between devices.
Interfaces are used to transmit data between personal computers and peripheral equipment, such as audio and video equipment or digital cameras. The interfaces of connected apparatuses are connected to one another by cables that include power lines and data lines. There is a type of interface that supplies power from one apparatus to another apparatus via the personal computer regardless of whether the system power of the personal computer is off. Among such interfaces, the IEEE1394 standard interface has attracted much attention.
FIG. 1 is a block diagram showing a data transmission device 100, which is provided with an interface. The data transmission device 100 is incorporated in a personal computer. Further, the data transmission device 100 includes two connectors 51, 52, a host controller 54, and an interface circuit 59, which has an interface (I/F) controller 55, two regulators 56, 57, and a diode 58. A system power circuit 53 is also incorporated in the personal computer.
The first connector 51 is connected to a first peripheral apparatus (not shown, in this case, a digital camera) via a first cable 61. The second connector 52 is connected to a second apparatus (not shown, in this case, a color page printer) via a second cable 62. Each cable 61, 62 is a six line cable having a positive power line, a ground power line, and four signal lines.
When the power of the personal computer is turned on, the host controller 54 is supplied with system power VDDsys which is generated by the system power circuit 53. Furthermore, the I/F controller 55 is supplied with operation power VDDif by way of the regulator 56, the diode 58, and the regulator 57. When the power of the personal computer is cut off, the supply of system power VDDsys from the system power circuit 53 is stopped. This opens the host controller 54.
If the digital camera and the color page printer are activated in this state, the regulator 57 is provided with cable power VDDcab from the digital camera via the first cable 61 and the first connector 51 (or from the color page printer via the second cable 62 and the second connector 52). The regulator 57 generates the power supply VDDif based on the cable power VDDcab and activates the I/F controller 55 with the power supply VDDif. Accordingly, even if the power of the personal computer is off, the I/F controller 55 receives data from the digital camera via the cable 61 and the first connector 51 and transmits the data to the color page printer via the second connector 52 and the cable 62.
However, if the power of the personal computer is turned on when the I/F controller 55 is performing data transmission, power supply noise mixes with the system power VDDsys. The power supply noise also mixes with the operation power VDDif of the I/F controller 55. As a result, the I/F controller 55 may function abnormally and affect the transmission of data in an undesirable manner.